How can I get only textures (with out shaders) from rendering image in Blender 2.8?
1-You can enable color pass in view layer settings. And access it in compositing.

2-You can do screenshot of the solid with right size by doing Viewport render image.
Its in view menu of viewport.

3-About 


  Just use the texture color directly to the surface output, with no
  shader in between. – lemon


With Node wrangler addon shift-click on texture in material editor to show just texture.
Or connect texture to emission shader and this shader to output node.